The determination of the softening- or dropping point is a commonly recognized procedure for the determination of characteristic properties of a material or a substance, specifically for substances that lack a sharply defined phase transition between the solid and the liquid state, making it impossible to determine an exact melting point.
The term “dropping point” means the temperature at which a substance begins to flow under given test conditions which are in many cases standardized. In the test, the substance is heated under controlled conditions until it changes from the solid to the liquid state. In the measurement, the temperature is registered at which a first drop detaches itself from the substance. The determination of the dropping point can be performed according to one of the common standards such as for example ASTM D3954, Ph. Eur. 2.2.17 (Pharmacopoeia Europaea), AOCS Cc 18-18 (American Oil Chemists' Society). In particular for polymers, raw polymers, waxes, polyolefins, paraffins, lubricant greases, organic powders, petroleum jellies, edible fats and oils and related substances, the dropping point is used for characterization as well as for quality control.
The softening point is a measured value used for the classification of, e.g., bitumina and bituminous substances. The softening point can be determined by means of a so-called ring-and-ball method as described for example in U.S. Pat. No. 3,242,277 A. For the determination of the value of the softening point, a steel ball is put on a layer of bitumen or a bitumen specimen positioned within a ring. In the course of the experiment, the material is uniformly heated, and the temperature at which the specimen has sagged downwards by 25.4±0.2 millimeters is recorded as the softening point. The softening point can also be determined by measuring the temperature at which an expanding drop has reached a certain length. The drop formation of the substance to be analyzed occurs as a result of heating. The softening point can be determined according to one of the common standards as for example ASTM D3104, D3461 and D6090 as well as DIN 51920. The determination of the softening point is likewise used for the quality control as well as for the characterization of bitumen, pitch, asphalt, resins, binding agents and related substances.
As an example, a device for the optical determination of the melting point is disclosed in GB 2464717 A, wherein a sample in a melting point capillary is visually observed during the heating of the sample and the data are evaluated for the determination of the melting point. The term “melting point” means the temperature at the phase transition from solid to liquid and lends itself well to visual observation, as a solid sample is normally opaque while a liquid sample is frequently transparent, so that the point at which an intensity maximum is reached can be essentially equated to the reaching of the melting point.
US 2009/0190626 A1 likewise discloses a device for the optical determination of the melting point and of other phase transitions of one or more substances that are arranged in depressions of a flat carrier. This device is less suitable to determine absolute values for the phase transitions, but is primarily intended for the determination of comparative values for different substances.
The sample or substance that is to be analyzed can be liquid or solid as well as in an intermediate state. For example, the substance can be described as liquid, solid, waxy, bituminous, viscous, or pulverous.
At present, the known state of the art offers primarily measuring instruments that allow either the determination of the softening point or the determination of the dropping point. In many cases, the determinations take place in measuring instruments with a heatable sample chamber. The substance to be analyzed is filled into a suitable sample receptacle which is set into the sample chamber in which the sample, i.e. the substance in the sample receptacle, is subsequently heated.
The reaching of the softening point is determined as the point at which the first drop of the substance has reached a given length and triggers for example an appropriately arranged light gate. The temperature at this time is registered as the softening point.
Another instrument for the optical determination of the softening point is disclosed in JP 58062551 A, wherein the shadow is measured that is cast by the drop and its length is evaluated.
The point at which the dropping point is reached is determined as the time at which the first drop falls off the sample. This event can likewise be detected by means of a light gate, and the corresponding temperature can be registered as the dropping point.
The measuring chambers of state-of-the-art measuring instruments are in most cases enclosed and not accessible for direct visual inspection. The enclosed design serves primarily to avoid thermal bridges, in particular cold bridges, reaching through a window and is chosen in the interest of a uniform heating of the sample. Other state-of-the-art measuring instruments use oil baths or water baths for the heating of the sample.
However, the determination of the softening point according to the conventional standards can only performed with substances whose behavior also conforms to those standards, i.e. substances which, when heated, will form a drop of the desired length. For example, no softening point can be determined for a substance even though a drop is formed by the heating, if the drop separates itself before it reaches the desired length or if the drop does not attain the prescribed length but hardens beforehand.
Consequently, the objective presents itself to provide a method as well as a measuring instrument which improves the reproducibility in the determination of the dropping- and/or softening point of substances conforming to the applicable standards and which, furthermore, makes it possible to also analyze substances whose behavior does not conform to the standards.